1.4 Impact on RF video 4 of 5: Local oscillator and GHz clocks requirements in Radio applications systems with LMX2594.

Hello, and welcome to video number four of the presentation, Local Oscillator and Gigahertz Clocks Requirement in Radio Applications Systems with LMX2594. My name is Simon. And in this video, we'll cover the section, how signal source specification impact RF radio performance. And more specifically, we'll cover the following five subtopic-- reciprocal mixing, signal to noise ratio and noise figure, error vector magnitude, coherent receiver, and phase alignment, and finally lock time. You are currently watching video number four. There are four more videos. And the topic of each of these videos is described here.
How does a signal source, and its specification impact RF radio performance? In order to discuss this topic, I would like to imagine a radio receiver example. This receiver is a single conversion with a data converter, same thing in the second Nyquist. And it has an RF frequency of 4,000 megahertz. The single is down converted to an IF of 375 megahertz. And the data converter is operating at 500 mega symbols per second.
In order for that single chain to work, we need two signal sources. We need a first single source at 3,625 megahertz. And that is the local oscillator. And we need a second signal source at 500 megahertz, which is literally the clock for the analog to digital converter. Both of these signal sources are driven from a common reference. And it is the LMK device here from Texas Instruments.
So what we're going to do now is, at fairly high level, describe how does phase noise, spurs, jitter, phase alignment, and lock time impact the performance of that receiver or multiple instances of that receiver. So the first topic to discuss here is reciprocal mixing. I take the mixer that is in the previous single chain I isolate it here. I place spectrum of each point on the RF side, on the LO side, and the IF side. So you can see the signal at 4,000 megahertz being down converted to 375 megahertz with the local oscillator at 3,625 megahertz.
You quickly notice here that the LO is having a little skirt which is a representation of phase noise. By doing the down converting process on a LO side injection configuration, we can see that the phase noise of the LO is superimposed now onto the IF signal. We could describe this with [? mats, ?] but simply put the mixer is literally chopping the RF at the LO rate. And any noise onto that LO is actually superimposed onto what is left, which is the IF frequency. So the LO phase noise translates into the IF and is superimposed onto the signal.
Now, if the quality of the remaining signal on the IF is not a problem, meaning the application does not need a certain level of integrity, that is fine. And here we have a receiver-- if we go back to the receiver on the previous slides, we have a receiver of about 3 dB noise figure. And our receiver sensitivity is fairly good. We have a low noise amplifier of 2 dB, an input filter of about half a dB insertion loss. And the rest of the chain probably contributing with that [? LNE ?] gain to about half a dB. So this 3 dB noise figure is rather good.
Now where things gets different is that if we add a blocker in band. By adding a blocker at minus 30 dBm here, what we see is that the phase noise that is on the LO gets also superimposed onto that blocker, and literally floods the desired signal area with noise from that phase noise. So what is the level of that noise? Well, the level of that noise is minus 30 dBm. And we know that a 100 kilohertz offset, that level is minus 120 DBC per Hertz. That means that the noise level at the desired frequency is now minus 150 dBm per Hertz. And that translates into a noise figure of about 24 dB.
So this is very different now. We have a receiver without blocker can achieve 3 dB noise figure. And as soon as we have a blocker, due to the phase noise on the LO, we have a noise figure of now 24 dB. So the thing to know here is that in order to improve this, either the blocker needs to be filtered out, which is not always possible, or the LO needs to be improved. There is no other technique here to improve the noise level in this signal chain.
The next topic to cover is spur. And remarkably, it is using the same concept. So if I have-- again, duplicating my mixer here with the spectrum for each signal-- I can see that I have a local oscillator here at roughly 3,625 megahertz. And I have a spur here at certain offset. That spur can be mixed with the mixer. And let's imagine simply that we have a blocker. And that blocker will mix again and superpose its mixing product with the spurious response here, over my desired signal.
So conceptually, the same thing as reciprocal mixing. If you have a spur into the LO drive and you have a blocker at a certain frequency that corresponds to that offset, you will have a response at the desired signal and here reducing your ability to decode your signal or reducing the integrity of the signal. So that is reciprocal mixing for phase noise, and also spur. The receiver sensitivity change drastically and the phase noise of the source is key. Phase noise and spur is key in down converting the signal without adding noise.
The next topic to discuss is the signal to noise ratio and the noise figure in the analog to digital converter. So from a data converter-centric perspective there would be a whole lot more to discuss here. But the point I want to drive is that the quality of the source impacts the signal to noise ratio that can be achieved with the data converter. So simply put, the signal to noise ratio is a function of the input frequency and the total jitter contained in that signal source.
Now remember we discussed in video two that the total jitter depends on the application and its integration limit. So you remember the slide where we had different applications require different integration limits. So here you need to go and set your integration limit to obtain the relevant total jitter. Once you know your input frequency you can make an assessment of the signal to noise ratio you'll be able to obtain with that signal source.
On the right hand side, I have a graph of the signal to noise ratio that can be achieved with an RMS jitter showing at different input frequencies. So if we were to be at 1 gigahertz and 100 femtosecond RMS jitter, we could achieve roughly 65 dB signal to noise ratio. And this sort of graph is interesting to see what kind of RMS jitter you can afford on to your data converter so that you do not impact the data converter's achievable signal to noise ratio. Now that you have the signal to noise ratio, you want to know what is the equivalent noise figure of that data converter so that you can place it into your signal chain and calculate a total noise figure and do the right thing in calculating the right trade-off for gain and linearity.
So the noise figure calculation is very simple. If your data converter is terminated, and typically terminated in a differential input, it will deliver a certain amount of power which corresponds to a voltage swing across this resistor. For the simplicity of the calculation here, I assume a full scale power of zero dBm. I assume a signal to noise ratio of 65 dB. And I assume assembling frequency of 500 mega symbols per second.
So what is the ADC noise figure, given these three parameters? Well, I take the total power that I can deliver to the ADC. I subtract the signal to noise ratio, which will give me the integrated noise that is on that full Nyquist zone on the analog to digital converter. And then I subtract 10 times the log of that Nyquist zone, so 250 megahertz. That will give me the amount of dBm per Hertz or power spectral density of my analog to digital converter. If I add 174 to this, I get the actual noise figure of the data converter which here is 25 dB. 25 dB seems to be a lot, but that's a typical number. And once the chain is properly put together, this sort of noise figure is nothing to worry about.
Now I would like to move to the next topic, which is the error vector magnitude. An error vector magnitude is the ability for the receiver to do not add phase uncertainty onto the signal. Let's imagine we have an RF signal where the information is placed on the phase. Let's say that it's a quadrature or QPSK here with four phase. And we have the symbol represented here. One symbol, two, three, four symbols.
The information is indeed on the phase of the signal. Well, we know that when down converting that signal with the local oscillator, all the phase noise contained in the local oscillator will superimpose onto that IF signal. If the phase noise of the LO is excessive, it will reduce the accuracy of that symbol to a point where it becomes a large impairment that is not acceptable.
So the phase noise on the LO, simply put, reduces the phase accuracy onto the modulation and create some halo or some region of phase uncertainty onto the symbol. The integration limit here, as we discuss in video number two, depends on the clock recovery loop on the receive side and on the symbol rate or the data rate for the actual communication system. And that is the error vector magnitude.
The next topic to discuss is coherent receiver and phase alignment. There are multiple applications either for multiple input, multiple output system or for phased array antenna where the phase or the integrity of the phase of the signal is critical to the system. So let's imagine here I have two channels and I place two mixers here. And each of these channels-- channel X and channel Y-- are driven with a local oscillator. And that local oscillator has different phase. So you can see here that the blue time domain sine wave that is here is having obviously different phase in time. The impact of having different phase on the local oscillator will actually change the phase on the IF.
So for the purpose of illustration here only, I rotated the actual constellation to show a different phase. But really, for example, in a phased array antenna where we would combine these signals, having the phase to be aligned, even before we talked about recovery or data recovery or demodulation, is critical. The interesting point here is that LMX2594 can provide deterministic phase from multiple instances of the device or from power up to power up, provide always the same phase to a first order. The two blocks that require synchronization to enable the deterministic phase is the output divider and the delta sigma modulator. And LMX2594 has the ability or the features to provide that synchronization so that you have deterministic phase.
The next topic to discuss is lock time. So here I imagine two applications where lock time is critical. One of them is in a communication system where there is frequency hopping. So frequency hopping is a communication system that don't like to, let's say, stay at a certain frequency for a long time. So it's constantly changing frequencies. So if the lock time is important or the repositioning of the local oscillator is important, that down time here is timed at you are not able to do anything. You need to wait for the local oscillator to reposition. So the smallest that time is, the better. So that's one application.
The other application is simply in tests. So let's say you have a piece of equipment that is doing a certain number of tasks and needs to change frequency. The faster it can change to these frequencies, do the work it needs to do, the better. So repositioning a certain local oscillator or a certain clock and waiting is downtime or wasted time. So to that end, LMX2594 can calibrate its entire VCO system in less than 25 microseconds. There are multiple methods to calibrate LMX2594. And each of them can reduce that calibration time. If you were to pre-calibrate LMX2594, you could enable the calibration and complete a lock in less than five microseconds.
That is the end of the section on how a signal source specification impact radio performance. I'm going to stop here. And we'll resume on video number five, which we'll discuss the data sheet specification and using a LMX2594 device.

Details

This training is discussing the key requirements of local oscillator in microwave/RF and GHz clocks in radio applications. The topics covered will establish the key relationships between the requirements of a signal source and their impact in a radio system. After this training, you will be armed with the ability to understand the system requirements of your customer and how they pertain to a signal source so you can engage with your customer in a very meaningful way. We will make clear simple links between phase noise and error vector magnitude (EVM) / reciprocal mixing, spurs and undesired responses, Jitter and SNR / noise figure, Synchronization and coherent receivers / phase array antenna, Sysref and deterministic latency. LMX2594 is a high performance signal source targeted at these applications with purpose built features.